Hydrocarbons can catalytically be converted with steam to obtain synthesis gas, i.e. mixtures of hydrogen (H2) and carbon monoxide (CO). As is explained in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1998 Electronic Release and 6th edition 2003, keyword “Gas Production”, this so-called steam reforming (steam reforming) is the most frequently used method for the production of synthesis gas, which subsequently can be converted to further important basic chemicals such as methanol or ammonia. Although it is possible to convert different hydrocarbons, such as for example naphtha, liquefied gas or refinery gases, the steam reforming of methane-containing natural gas is dominant.
The steam reforming of natural gas proceeds strongly endothermally. It is therefore carried out in a reformer furnace in which numerous catalyst-containing reformer tubes are arranged in parallel, in which the steam reforming reaction takes place. The outer walls of the reformer furnace as well as its ceiling and its bottom are lined or covered with several layers of refractory material which withstands temperatures up to 1200° C. The reformer tubes mostly are fired by means of burners, which are mounted on the upper side or bottom side or at the side walls of the reformer furnace and directly fire the space between the reformer tubes. The heat transfer to the reformer tubes is effected by thermal radiation and convective heat transfer from the hot flue gases.
After preheating by heat exchangers or fired heaters to about 500° C., the hydrocarbon-steam mixture enters into the reformer tubes after final heating to about 500 to 800° C. and is converted there at the reforming catalyst to obtain carbon monoxide and hydrogen. Nickel-based reforming catalysts are widely used. Whereas higher hydrocarbons are completely converted to carbon monoxide and hydrogen, a partial conversion usually is effected in the case of methane. The composition of the product gas is determined by the reaction equilibrium; beside carbon monoxide and hydrogen, the product gas therefore also contains carbon dioxide, non-converted methane and steam.
For energy optimization and/or for feedstocks with higher hydrocarbons, a so-called prereformer can be used after the preheater for pre-cracking the feedstock. In a further heater, the pre-cracked feedstock then is heated to the desired inlet temperature into the main reformer, for example the steam reformer.
Pre-reforming mostly is understood to be the application of an adiabatic low-temperature reforming step, which is arranged upstream of a conventional steam reformer operated with natural gas. The conventional pre-reforming can be defined as steam reforming process at limited temperatures (distinctly below 700° C.). It leads to a gaseous intermediate product whose main constituents are methane and steam. The intermediate product contains no or only small amounts of higher hydrocarbons. This intermediate product normally is treated further in a steam reformer referred to as main reformer.
In contrast to the steam reforming reaction, the reaction equilibrium is set at far lower temperatures during pre-reforming. The main feature of the pre-reforming is the irreversible, complete conversion of the higher hydrocarbons in the feed mixture to obtain methane and in part synthesis gas constituents. Due to the distinctly lower temperature as compared to the steam reforming, the main product of the pre-reforming is methane beside non-converted steam. The remaining gas components are hydrogen, carbon dioxide, traces of carbon monoxide and inert components which have already been present in the feedstock. In dependence on the feedstock, pre-reforming can proceed endothermally or exothermally. In general, the steam conversion of the hydrocarbons to carbon monoxide and hydrogen is endothermal. But since pre-reforming is carried out at moderate temperatures only, the produced carbon oxides partly are converted further to obtain methane, a reaction with considerable exothermicity. For this reason, pre-reforming of naphtha is an exothermal process, whereas pre-reforming of natural gas leads to a generally endothermal course of the pre-reforming reaction.
Since pre-reforming is a steam reforming process at low temperatures, special catalysts are required, in order to provide for sufficiently high reaction rates. In general, this is achieved by means of catalysts which have a high nickel content, for example 30 to 70 wt-%.
Of the various possibilities of using the pre-reforming process, pre-reforming of natural gas presently is utilized most frequently. The driving force for the application of this technology is the general endeavor to attain a method with improved economy. The prereformer is mounted upstream of a main reformer including a plurality of catalyst-filled tubes, in order to simplify the operation of the main reformer. Since virtually all higher hydrocarbons which are present in the natural gas used as feed are converted to methane and synthesis gas constituents, the risk of the formation of coke deposits in the main reformer, which with respect to the operation of the main reformer represents a particularly critical point, is reduced considerably. This permits the decrease of the steam/carbon ratio (S/C) and the increase of the heat load of the reformer tubes, which leads to a generally lower energy consumption and to a reduction in size of the used apparatuses. In addition, an amount of hydrogen already is produced in the prereformer by conversion of natural gas, and traces of catalyst poisons left in the feed mixture are adsorbed or absorbed on the pre-reforming catalyst. This leads to the fact that the reforming catalyst present in the main reformer operates under optimum conditions in particular at its inlet.
Pre-reforming generally is operated in adiabatic shaft reactors, which have a typical inlet temperature in the vicinity of 500° C. Due to the endothermal conversion of the hydrocarbons, the temperature at the outlet of the pre-reforming reactor typically is lower by 25 to 40° C., in dependence on the amount of the higher hydrocarbons in the natural gas. The gas product leaving the pre-reforming stage, mostly is heated up further before being introduced into the main reformer. Since the steam reforming technology generally has an excess of energy, which otherwise can only be used for the production of process steam or export steam, this is an additional possibility for improving the total energy balance of the integrated reforming plant. In the conventional steam reforming process, the reintegration of the available process heat either by recirculation of the flue gases of the reformer furnace or by recovery from the product gases is limited by the risk of cracking, which occurs when natural gas/steam mixtures are heated up to temperatures above about 550° C. This risk rises considerably with increasing content of higher hydrocarbons. Due to the absence of all higher hydrocarbons with the exception of methane and the higher hydrocarbon content, prereformed natural gas can be heated up to temperatures of about 650° C. without significant cracking.
From the prior art, further developments of the above-discussed basic concept of the use of the pre-reforming also are known, which aim at operating the pre-reforming in higher temperature ranges.
The European Patent Application EP 1241130 A1 for example discloses a method for producing a synthesis gas, in which a desulfurized light natural gas is mixed with steam and preheated, a first reforming reaction is carried out at a temperature of 500 to 750° C. under adiabatic conditions, in that the gas mixture is brought in contact with a catalyst with defined porosity, which in addition has a specific content of nickel or a metal of the platinum group as active component on a carrier, consisting of CaO/Al2O3 mixtures or α-Al2O3. The catalyst has a nickel content of 3 to 20 wt-% or a content of the metal of the platinum group of 0.2 to 5 wt-%. Subsequently, a further reforming (main reforming) is carried out in a reformer furnace comprising reformer tubes. The light natural gas can be obtained from a heavy natural gas containing higher hydrocarbons by conversion in a reactor which contains a catalyst active for the methanation of carbon oxides, i.e. by a further pre-reforming stage upstream of the pre-reforming taught here, which is operated at inlet temperatures of 350 to 450° C. Therefore, a total of two pre-reforming stages and one main reforming stage is obtained.
The International Patent Application WO 2010/120962 A1 describes a further, likewise two-stage pre-reforming method, in which the feed mixture containing steam and hydrocarbons of a first adiabatic reaction stage operated at temperatures of 425 to 600° C. is converted on a first reforming catalyst, which has a content of 30 to 50 wt-% of a metal from a group comprising nickel and cobalt on a carrier. In the second pre-reforming stage downstream of the first pre-reforming stage, the further conversion is effected after heating to temperatures between 425 and 730° C. on a first reforming catalyst, which has a content of 10 to 20 wt-% of a metal from a group comprising nickel and cobalt on a carrier. The second pre-reforming stage likewise can be operated adiabatically, but can also be heated. The product of the two-stage pre-reforming is supplied to a downstream main reforming stage comprising a plurality of catalyst-filled reformer tubes, which is arranged in a reformer furnace.
What is disadvantageous in the method for pre-reforming at high temperatures, i.e. above 600 to 650° C., which is described in the prior art, is the limitation of the conversion on catalysts optimized for the use at such high temperatures due to the adiabatic reaction control. These catalysts have a high stability and sufficient conversions at high temperatures, which is achieved by limiting the nickel content to contents of typically below 30 wt-%. In the adiabatic conversion, however, the gas temperature decreases in flow direction, due to the endothermal conversion of the methane to carbon oxides and hydrogen, so that the optimum temperature range for the high-temperature pre-reforming catalyst is left in downward direction. The consequence is that in the portion of the pre-reforming reactor close to the outlet only insufficient hydrocarbon conversions are achieved, whereby the space-time yield based on the reactor volume is limited. On the other hand, adiabatic reactors as compared to heated reactors have the advantage of the constructive simplicity, which leads to lower apparatus costs and a greater ruggedness of the reactors used.